Predef.identity 在 Scala 中有什么作用? [英] What does Predef.identity do in scala?

问题描述

这里是关于的文档Predef，但没有关于身份的字眼.这个函数是干什么用的?它有什么作用?

Here is documentation about Predef, but there is no word about identity. What is this function used for? And what it does?

推荐答案

这只是身份函数的一个实例，预定义是为了方便，也许是为了防止人们自己多次重新定义它.identity 只是返回它的参数.有时传递给高阶函数会很方便.你可以这样做:

It's just an instance of the identity function, predefined for convenience, and perhaps to prevent people from redefining it on their own a whole bunch of times. identity simply returns its argument. It can be handy sometimes to pass to higher-order functions. You could do something like:

scala> def squareIf(test: Boolean) = List(1, 2, 3, 4, 5).map(if (test) x => x * x else identity) 
squareIf: (test: Boolean)List[Int]

scala> squareIf(true)
res4: List[Int] = List(1, 4, 9, 16, 25)

scala> squareIf(false)
res5: List[Int] = List(1, 2, 3, 4, 5)

我也看到它有时用作默认参数值.显然，你可以直接说 x =>;x 任何你可能使用 identity 的地方，你甚至可以保存几个字符，所以它不会给你带来太多好处，但它可以自我记录.

I've also seen it used as a default argument value at times. Obviously, you could just say x => x any place you might use identity, and you'd even save a couple characters, so it doesn't buy you much, but it can be self-documenting.

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